Various types of punch press tools are known in order to punch holes into a strip of sheet metal, and to derive punched blanks from the strip of sheet metal. Such punch press tools are used frequently in automatic punch presses and punching machines. The usual machines of this type have elements to guide a strip therethrough, and a reciprocating punch press tool is guided over the strip into a die in predetermined position, for example to punch a sequence of holes in the marginal portions of a band or tape of the metal strip. The desired result may be, either, the punching of holes into the metal strip or to obtain punched blanks.
Punch presses of this type have the problem that the punched blanks must be reliably carried away from the punch die, for example through a longitudinal bore therein, as soon as the punch press tool reciprocates. This problem arises regardless of whether the punched blanks are to be considered scrap--for example when the purpose of the tool is to punch a sequence of holes into the metal strip--or if the punched blanks themselves are to be the articles desired by the punching operation. Usually, the tool, and hence the punched blanks, will--in cross section--be circular.
Punching tools of this type, in which blanks are to be punched in excess of several millimeters diameter, frequently use a pressure element located centrally within the reciprocating punch press which, upon or after the punching operation, pushes the punched blank away from the surface of the punch press tool. This element, for example, is a spring or the like. When the size of the holes, and hence the size of the blanks, becomes very small, for example with circular diameters of 3 mm, 2 mm, or less, difficulties arise since it is no longer possible to place push-away or similar elements within the punch tool itself. As the punched blanks become smaller, however, the tendency of the blanks to adhere to the end face of the punch tool increases, primarily due to their light weight. Adhesion of punched blanks is additionally difficult to control if the surfaces of the strips from which the holes are punched are slightly adhesive, for example by having a thin coating of grease or the like. Various attempts have been made to prevent adhesion of the punched blanks to the punch press tool by special shaping of the cutting surface of the punch press tool. For example, it has been proposed to form the reciprocating punch tool with concave surfaces, or with converging surfaces, primarily in order to shape the punched blanks to have some angled portions, in order to prevent jamming of the punched blanks in the bore of the punching die and, further, to lift off the reciprocating punch press due to the springiness of the punched blank as it snaps back from the angled or bowed position to a flat position. Making the punch die in this shape, however, decreases its stability and, particularly if the punch die is of indented or roof-like shape, causes rapid wear of the cutting edge portions which project beyond other portions.
It has been attempted to decrease the cutting gap between the bore of the die and the reciprocating punch press tool. Closer tolerances of the tool-die combination, however, increases the wear on both of the tool elements, and thus decreases the effective operating time of the punch-die combination before re-grinding or re-surfacing of the elements is required. Further, the power requirements to effect punching are increased.